The invention relates to a method for driving at least one capacitive control element, in particular a piezoelectric control element for a fuel injection valve in an internal combustion engine. In the process, the capacitive control element is driven using a control circuit. The control element is charged with a nominal voltage applied to a series circuit comprising a charge capacitor and a charge reversal capacitor and is discharged into the charge reversal capacitor, which is charged to a prescribed voltage value in the process. The invention also relates to an apparatus for carrying out this method.
U.S. Pat. No. 5,691,592 discloses an apparatus for driving at least one piezoelectric control element in which the control element is charged from a series circuit comprising a charge capacitor, which is charged to a nominal voltage using a voltage source, and a charge reversal capacitor and is subsequently discharged into the charge reversal capacitor. As soon as the charge reversal capacitor has reached a prescribed voltage, the residual charge of the control element is nullified by means of a shunt regulator.
German published patent application DE 196 32 837 A1 discloses a method and an apparatus for driving at least one piezoelectric control element for a fuel injection valve in an internal combustion engine. There, the control element is charged from a series circuit comprising a charge capacitor, which is charged to a nominal voltage using a regulatable voltage source, and a charge reversal capacitor and is subsequently discharged into the charge reversal capacitor. The charge capacitor is subsequently recharged such that the voltage on the series circuit comprising the charge capacitor and the charge reversal capacitor is equivalent to the nominal voltage.
German patent DE 196 32 871 C2 discloses a method and an apparatus for driving at least one piezoelectric control element for a fuel injection valve in an internal combustion engine. There, the control element is charged from a series circuit comprising a charge capacitor, which is charged to a nominal voltage using a regulatable voltage source, and a charge reversal capacitor and is subsequently discharged into the parallel circuit comprising the charge capacitor and the charge reversal capacitor. The charge capacitor is subsequently recharged such that the voltage on the series circuit comprising the charge capacitor and the charge reversal capacitor is equivalent to the nominal voltage.
The two latest-mentioned methods work in internal combustion engines only in the steady-state, settled condition for a particular operating point of the circuit, without any stochastic reaction of the control element on the drive circuit, caused by dynamic state changes in the internal combustion engine or in a motor vehicle driven by the latter, such as:
rotational speed fluctuations upon starting and switching off and when the internal combustion engine changes from and to idling;
changing from overrun operation to regular operation, and vice versa;
when the interval between preliminary injection and main injection is varied;
during stochastic post-injections; and
when the charge fed back from the control element into the charge reversal capacitor is varied.
The reactions resulting from such dynamic state changes affect the voltage on the charge reversal capacitor (that voltage is also dependent on the control element capacitance, the engine speed, the fuel pressure, etc.) during charge reversal and causes changes in the charge reversal capacitor, in particular increases in the level of the charge energy and hence of the voltage across the charge reversal capacitor.
As a result of this, the voltage across the charge capacitor must also change, in particular be reduced, in order to be able to follow these dynamic changes and in order to keep the aggregate voltage across the charge capacitor and the charge reversal capacitor constant for the next time the control element is driven; this is not possible as quickly as would be necessary, however. Reducing the voltage across the charge capacitor by limiting the voltage source would take too long. Rapid discharge of the charge capacitor could be achieved only with additional outlay on a plurality of switches (which would also result in additional power loss) and on complex driving software.
The object of the present invention is to provide a method and apparatus for driving a capacitive actuator which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, such that the prior apparatus a method are improved the known apparatus and the method to the extent that they are able to compensate for the described reactions of the control element on the drive circuit during dynamic state changes.
With the above and other objects in view there is provided, in accordance with the invention, a method of driving at least one capacitive control element with a control circuit. In a preferred mode, the control element is a piezoelectrically operated fuel injection valve in an internal combustion engine. The method comprises:
providing a series circuit of a charge capacitor and a charge reversal capacitor, the charge capacitor having a capacitance substantially greater than a capacitance of the charge reversal capacitor;
selectively charging a capacitive control element with a nominal voltage applied to the series circuit of the charge capacitor and the charge reversal capacitor, and discharging the control element into the charge reversal capacitor, and thereby charging the charge reversal capacitor to a prescribed voltage value; and
when the charge reversal capacitor has reached the prescribed voltage value, charging a remaining residual charge of the control element into the charge capacitor.
This object is achieved in accordance with the features of patent claim 1 by virtue of the fact that the charge reversal capacitor (C2) has much smaller dimensions than the charge capacitor C1 (C2 less than  less than C1) and, when the control element is discharged, charging is carried out only up to a prescribable voltage value, and that the excess residual charge of the control element is discharged into the charge capacitor C1, on which it produces only an insignificant, negligible voltage swing on account of the capacitance ratio C1 greater than  greater than C2 between the charge capacitor and the charge reversal capacitor. In this manner, reactions of the control element on the drive circuit during dynamic state changes are compensated for.
In accordance with an added feature of the invention, the prescribed voltage value to which the charge reversal capacitor is charged when a control element is discharged corresponds to a sum U2=U1+Uz of a voltage U1 on the charge capacitor and a breakdown voltage Uz of a Zener diode connected in series with the charge capacitor.
In accordance with an additional feature of the invention, the control circuit contains a value (either formed in the control circuit or fed to it) for the nominal voltage of each control element, and the charge reversal capacitor is charged, when a previously driven control element is discharged, to a voltage associated with the control element in accordance with the formula U2n=Usnxe2x88x92U1, where n=2. . . x, Usn is the nominal voltage of the respective control element, and U1 is the voltage of the charge capacitor.
With the above and other objects in view there is also provided, in accordance with the invention, an apparatus for performing the above-outline method, i.e., for driving at least one capacitive control element. The apparatus according to the invention comprises:
a charge capacitor connected between a positive pole and a negative pole of a voltage source;
a first series circuit connected in parallel with the charge capacitor, the first series circuit including a charging switch and a discharging switch connected to the negative pole;
a second series circuit connected in parallel with the discharging switch, the second series circuit including a charge reversal capacitor, a charge reversal coil, a first control element, and a first selection switch;
for each further control element, a further series circuit connected in parallel with the first control element and the first selection switch, each the further series circuit respectively including a control element and a selection switch associated with the control element;
a control circuit connected to and controlling all the switches;
wherein the charge capacitor is dimensioned substantially larger than the charge reversal capacitor;
a limiter path connected from a node between the charge reversal capacitor and the charge reversal coil to a node between the charge capacitor and the charging switch, the limiter path comprising a series circuit formed of a diode, which is forward biased in a direction of the charge capacitor, and a limiter switch.
In accordance with another feature of the invention, the limiter switch is turned on as soon as the charge reversal capacitor is charged, when a control element is discharged, to a voltage associated with a next control element to be charged.
In accordance with a further feature of the invention, the limiter path further includes a Zener diode between the diode and the limiter switch in a reverse direction towards the charge capacitor.
In accordance with again a further feature of the invention, the limiter switch is turned on and off synchronously with the discharging switch.
In accordance with a concomitant feature of the invention, the voltage source is a controlled voltage source and an output voltage of the voltage source can be regulated.
The expression xe2x80x9csubstantially greaterxe2x80x9d will be understood by those of skill in the art within the technological boundaries. As described herein, it may be understood, by way of example, as one order of magnitude and preferably even two orders of magnitude.
Of particular advantage is the fact that the known circuit need be expanded only by a few components in order to achieve this object, and that the voltage source may be a constant voltage source.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and apparatus for driving at least one capacitive control element, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.